NOx is released in combustion processes in, for example, gas engines. At present only very few gas engines are equipped with a deNOx installation. Apart from NOx, the exhaust gases of gas engines contain considerable amounts of uncombusted methane; sometimes up to 3% of the fuel leaves the engine uncombusted. These methane emissions must also be controlled as part of the reduction of greenhouse gas emissions.
NOx can also be released from gas burners in horticulture, generating sets, emergency power supplies, gas turbines of (small-scale) combined heat and power systems, and in the industrial production of, for example, cement, nitric aid, iron or caprolactam, in traffic and in the burning of household refuse.
There are various techniques on the market for reducing NOx emmisions, such as low-NOx burners and selective catalytic reduction with ammonia or urea. These techniques, however, are impossible or expensive to apply for many (small-scale) (gas burner) installations which produce NOx. There is therefore a demand for an inexpensive downstream technique for the reduction of NOx.
U.S. Pat. Nos. 5,149,512 and 5,260,043 describe methods in which NOx is removed with the aid of methane and in which inter alia a catalyst is used which consists of a ZSM-5 zeolite loaded with cobalt. This catalyst, however, only has limited activity for the catalytic reduction of NOx with methane. In the absence of water, temperature above 450° C. are necessary for an NOx removal efficiency above 50%. In the presence of water, however, it must be expected that the NOx conversion will decrease by about half.
According to the review article by Traa et al., Co-ZSM-5 can also be used for the reduction of NOx with propane (Y. Traa, B. Burger, J. Weitkamp, Micr. Mes. Mater. 30 (1999) 3-41). It was found here that the method of preparation of the catalyst was critical, and that much higher activities were obtained if the zeolite was loaded with cobalt by the impregnation method (incipient wetness).
A much more efficient catalyst for the reduction of NOx with methane was found in the form of ZSM-5 with palladium. It is true that these zeolites have a higher activity than zeolites based on cobalt, but it turns out that the activity of the Pd zeolite catalyst also decreases greatly in the presence of water. Loss of activity is also clearly observed as a function of time, (see for example Y. Traa, B. Burger, J. Weitkamp, Micr. Mes, Mater. 30 (1999) 3-41).
Ogura et al. (M. Ogura, S. Kage, M. Hayashi, M. Matsukate and E. Kikuchi, Appl. Catal. B 27 (2000), L213-216) describe the stabilization of Pd-ZSM-5 with the aid of inter alia cobalt, rhodium, silver, cerium or iron. It is apparent from his study that cobalt is highly suitable as a stabilizer. Cobalt might also have a promoting effect on the reaction (promotor). The other elements, rhodium, silver, cerium and iron, are promotors in the reaction and also provide Pd-ZSM-5 stabilization.
From FIG. 3 in this publication, however, it can be deduced that although these elements can have a stabilizing effect (the half-life increases), it also turns out that the initial conversion activity deceases, if loaded with rhodium the initial activity goes from 49.7% to 18.9% (drop of about 60%), with silver the initial activity goes from 49.7% to 29.9% (drop of about 40%), with cerium the initial activity goes from 49.7% to 39.6% (drop of about 20%) and with iron the initial activity goes from 49.7% to 40% (drop of about 20%). In a number of cases, this may mean that for a substantial part of its life the stabilized catalyst has a lower activity than the non-stabilized catalyst. That is not desirable. The fact that the initial conversions differ so much with the different metal combinations also makes it almost impossible here to check whether there is in fact any stabilizing effect at all.
Only for the Pd-ZSM-5 catalyst stabilized with cobalt (FIGS. 1 and 2 of this publication) does it appear to be true that the activity scarcely decreases as a result of the addition of (3.3% by weight) cobalt, while good stabilization is indeed obtained (in any case for a reaction time up to about 14 h). On the basis of this article the person skilled in the art would opt for a Pd—Co-ZSM-5 catalyst for the reduction of NOx using methane. The activity of this catalyst is limited, however: only 60% NOx conversion is achieved at 500° C.; the stability after about 14 h is not known.
In U.S. Pat. No. 6,063,351 a catalyst based on this palladium-cobalt pairing, with mordenite (MOR) as carrier, is described for the reduction of NOx with methane. This catalyst has markedly improved activity compared with the cobalt catalyst of the above-mentioned U.S. Pat. No. 5,149,512. Experimentally, however, the stability of this catalyst is found to be inadequate in the long term.
Japanese patent abstracts JP 09 192486, JP 08 164338 and JP 07 32325 also describe catalysts in which Pd may be present. However, none of these abstracts show or indicate that palladium should be present as an ion coordinated by a zeolite. In contrast, for example JP 08 164338 describes that Pd (oxide) layers are present on a zeolite, and JP 07 32325 describes that oxides are present on a γ-alumina substrate. The disadvantages of these catalysts are the same as the disadvantages of the other catalysts know from the art and described above.